a) Field of the Invention
This invention relates to filler treatment with swollen starch-latex compositions, prepared in the presence or absence of co-additives, for use in the manufacture of filled wood-free and wood-containing paper grades. The invention also relates to combinations of swollen starch and latex for use as furnish additives in grades that contain no filler such as sack papers and paperboard products.
b) Description of Prior Art
In the manufacture of filled papers the filler slurry is added to the pulp suspension before it is transferred to the forming section of the paper machine. A retention aid or retention aid system comprising several components, is always added to the pulp/filler suspension (also known as the furnish) to retain as much of the filler as possible in the sheet. Adding filler to paper provides the papermaker with numerous improvements in sheet properties, including improved opacity, brightness, feel, and print definition. Furthermore, when the filler is cheaper than the pulp, addition of filler to the sheet results in cost savings due to the replacement of the fibre by filler. These savings can be substantial when low cost fillers, such as precipitated calcium carbonate (PCC), are used to replace expensive chemical pulp fibres. Moreover, filled paper is much easier to dry than paper with no filler and, as a result, the papermachine can run faster with less steam consumption, which reduces costs and improves productivity. Therefore, the addition of high levels of PCC to the sheet would drastically reduce the cost of fine paper manufacture.
However, for a given sheet weight there are limits to the amount of filler that can be added. The strength of paper is usually the most important factor limiting the filler content, although other factors such as retention, drainage, and the chemical demand for retention and sizing are also a consideration.
Making paper with a high filler content requires an efficient retention aid system. It is required that the retention aid gives good filler retention under the high shear and turbulence levels found in the papermachine and that it improve drainage, but without impairing formation. The retention aid chemicals are added to the papermaking furnish, at a point prior to or at the inlet of the headbox of the paper machine. The retention aids are composed of single or dual chemical additives that improve filler and fines retention by a bridging and/or flocculation mechanism. The chemicals help attach the filler particles and fines (small fibrous fragments) to the long fibres or cause their aggregation into larger flocculated particles which are more easily retained in the web. In order to create the attachment and flocculation the chemicals must adsorb on the surfaces of the fillers, fines and fibres. The degree of adsorption of chemicals and the attachment forces are influenced by many things including furnish cleanliness and furnish chemistry, the properties of the added chemicals, the level of shear in the papermaking process and the contact time between the retention aids and the furnish components.
Paper strength is inevitably reduced by replacement of the fibres by filler; not only because there are less fibres in the sheet which reduces the number of fibre-fibre bonds in the sheet, but also because the presence of the filler reduces the area of contact between the remaining fibres. Filler particles do not bond between themselves and their location at the fibre-fibre bonded area prevents hydrogen bonding from occurring between the pulp fibres. As a result, retaining high amounts of filler produces a weaker sheet that can break more easily on the paper machine, size press, coater, winders and printing presses. Weaker fibre-fibre bonding also decreases the surface strength of the paper, causing a reduction in pick resistance and an increase in linting. Poor bonding of filler particles in the fibrous structure can also increase dusting in the pressroom.
In general, all common inorganic fillers, (for example, clay, ground calcium carbonate (GCC), PCC, chalk, talc, titanium dioxide, precipitated calcium sulphate, are known to impair strength and increase demand for chemicals. In particular, fillers with high surface areas, such as scalenohedral PCC which is widely used in the manufacture of fine papers, have excessive negative effects on strength and increase the chemical demand of additives used for strength, sizing and retention. Due to its shape, narrow particle size distribution, and high surface area, PCC has a tendency to reduce bonding in the sheet more than other common papermaking fillers, such as chalk, GCC and clay, and also gives the sheet an open structure which makes the sheet excessively permeable or porous. High sheet porosity is detrimental for print quality and liquid absorbency. As the content of PCC is increased in the furnish the demand for sizing chemicals, such as alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) is increased to maintain the desired degree of sizing or water repellence. This is because a disproportionate fraction of the sizing chemical is adsorbed on the high surface area PCC. Poor sizing efficiency and loss of water repellence over time (size reversion) are common problems associated with the use of PCC in highly-filled wood free papers sized with AKD and ASA. In recent years many paper mills making wood-containing grades have converted to neutral papermaking to allow use of bright calcium carbonate fillers, such as GCC and PCC, and major concerns with the use of PCC in these grades remains retention, sheet strength and printing operations.
An ongoing industry trend is to decrease sheet grammage to reduce costs. Unfortunately, as the grammage is decreased nearly all paper properties deteriorate, including the limiting factors of opacity, bending stiffness and permeability. Reduction in grammage may also decrease retention of filler during papermaking and increase the frequency of sheet breaks both on the paper machine and during converting and printing. To overcome the loss in sheet opacity the papermaker can add more of the high opacity fillers, but this in turn can cause further deterioration in sheet strength. The industry needs cost-efficient technology for the production of the lightweight grades with good filler retention and drainage and acceptable strength, formation, optical, and printing characteristics.
Water-soluble natural and synthetic polymers are commonly used for strength development in the manufacture of filled and unfilled paper grades. Starch is the oldest and most widely used additive for increasing the strength of paper. In order to increase strength the starch macromolecules must adsorb on the long fibres and reinforce the fibre-fibre bonded areas. Cationic and amphoteric starches are added to the paper machine wet-end in the production of coated and uncoated wood-free fine papers, bleached paperboards, and many filled- and unfilled-grades. Since starch is inexpensive compared to synthetic polymers its dosage level can be as high as 40 kg per ton or more. Cationic starch is also used in the preparation of dispersions of AKD, ASA and rosin sizes, and as a retention aid in combination with a silica micro-particle such as anionic colloidal silicic acid. The cationic starch or cationic starch-size dispersions are usually easily adsorbed on the negatively-charged fibres and fines and are retained in the sheet during the forming process. Unfortunately, when cationic starch is used in chemical pulp furnishes the improvement in the strength of the paper is often low and addition of higher levels of starch does not improve strength. This phenomenon is related to the limited amount of starch that can adsorb on the fibres. It appears once the negative charge on the fibre surfaces is neutralised by the cationic charge of the starch macromolecules, no further starch adsorbs, even at high dosage rates. With mechanical pulp furnishes the starch performance is usually reduced by the high levels of fibre fines and anionic colloidal solids. The anionic colloidal solids, also known as anionic trash or dissolved and colloidal substances (DCS), can neutralise a large portion of the cationic charge on the starch making it ineffective for improving fibre-fibre bonding. The application of starch in the manufacture of filled wood-free and some wood-containing grades is often limited to a maximum of 4 to 10 kg/ton of paper. At higher dosage rates the starch may impair drainage and other sheet qualities, such as formation, porosity and brightness, and the improvements in tensile strength are usually small. At present, there are no cost-efficient polymers capable of developing adequate strength when added to furnishes containing high levels of fines and DCS such as found in mechanical pulp furnishes.
Cationic starch is normally used as a papermaking additive after being fully cooked. Generally, the starch powder is dispersed in cold water at about 2%-6% concentration then cooked or gelatinized either in batch cookers at 96° C.-100° C. for a period of about half an hour or in jet-cookers at 120-140° C. for a few minutes. These cooking processes lead to complete gelatinization of the starch granules followed by their dissolution into amylose and amylopectin macromolecules. In special applications, such as in the manufacture of heavy paperboard products, the dispersed starch granules are also applied directly to the formed sheet by spraying the uncooked starch slurry onto the moist web. The gelatinisation of the starch granules is believed to take place during the drying operation of the sheet. An improvement in the starch cooking process for use in the manufacture of paper was disclosed more than forty years ago in U.S. Pat. No. 2,805,966, which describes the cooking of the starch slurry in a steam injection cooker. This was said to permit control of the heating so that the majority of the starch granules were swollen but not ruptured. Two other methods to produce a swollen starch whose granules do not disintegrate during agitation were disclosed in U.S. Pat. No. 2,113,034 and U.S. Pat. No. 2,328,537. In U.S. Pat. No. 2,113,034 this was accomplished by reaction of the starch with formaldehyde. In U.S. Pat. No. 2,328,537 this was accomplished by reaction of starch with certain antimony or phosphorous chlorides or oxychlorides. The patents suggest that the products might be useful in manufacture of paper. However, since the products have limited swelling characteristics even in hot water and are only partially retained in the paper sheet they never found acceptance in the paper manufacturing industry. U.S. Pat. No. 5,620,510 also discloses a method for preparation of a swollen starch for use as a dry strength additive in the papermaking process. In this invention the swollen granules of starch were produced under controlled conditions of temperature and pH that prevent their disintegration during agitation. An alternative method of producing swollen modified starch for increasing strength of paper was disclosed in patent WO 97/46591. The modified starch is prepared by a process comprising the step of swelling a cationised cross-linked starch under conditions selected so that a cross-linking agent, sodium trimetaphosphate, maintains the viscosity of the swollen product at a level less than 400 cps. The washed swollen product is to be added to the paper furnish, at or prior to the headbox of the paper machine. The swollen starches of all the above patents were proposed for addition to a paper machine pulp furnish.
It has been common knowledge in the paper industry that the addition of an anionic latex to a papermaking furnish, combined with alum (aluminum chloride), causes the latex to precipitate and thereby give increased strength to paperboard. A number of patents, particularly U.S. Pat. No. 4,178,205, U.S. Pat. No. 4,189,345, and U.S. Pat. No. 4,187,142, disclose the general idea that a cationic latex can be added to the papermaking furnish. Because of the anionic nature of the pulp furnish, cationic latex adsorbs easily on pulp surfaces and provide additional fibre-fibre bonding and tensile strength to the paper product. These patents relate primarily to so-called “high-strength” papers, which are largely made without addition of fillers. The furnishes of these paper grades contain many other additives including starch, size, alum, and retention aids. Therefore, the strengthening benefits from the addition of latex might be attributed to its interaction with these additives. In own laboratory handsheet studies on pulp suspensions containing no other additives, it has been found that at equal dosage levels the cationic latices are about 10 times less efficient than a cooked cationic starch in increasing strength. For example, an addition level of 1% cationic starch, on pulp, produced a greater internal bond strength and tensile strength than the dosage of 10% cationic latex, despite the large amount of latex being retained in the sheet. Similar low strength results were also obtained when anionic latices were added to pulp suspensions where the fibres have previously been made cationic to promote latex adsorption.
Another approach for improving filler retention, strength and sizing performance is by treating the filler suspension with additives prior to mixing with the pulp stock. For example, several patents including U.S. Pat. No. 4,225,383, U.S. Pat. No. 4,115,187, U.S. Pat. No. 4,445,970, U.S. Pat. No. 5,514,212, GB Patent 2,016,498, U.S. Pat. No. 4,710,270, and GB Patent 1,505,641 describe the benefits of filler treatment with additives on retention and sheet properties. It is known that since most common inorganic filler particles in suspension carry a negative charge, the cationic additive adsorbs on their surfaces by electrostatic interactions causing their agglomeration or flocculation. For anionic additives to promote flocculation the filler particles would require a positive charge to allow adsorption of the anionic additive. The flocculation of the filler particles usually improves retention during sheet making and also increases sheet strength, but excessive flocculation of filler can also decrease the gain in optical properties expected from the filler addition. GB Patent 2,016,498 discloses flocculating fillers with a composition comprising cooked starch, an organic polyelectrolyte, and an agent for controlling the degree of flocculation and viscosity of dispersion. The resulting pre-flocculated filler is disclosed to provide improved tensile strength in filled paper. U.S. Pat. No. 4,710,270 discloses pre-flocculated filler particles covered with a dispersion of cationic starch and carboxymethylcellulose or alginate, resulting in improved strength and retention.
GB Patent 1,505,641 discloses treating calcium carbonate filler with anionic styrene-butadiene latex. Pre-treatment of calcium carbonate filler, especially chalk whiting, with this latex is used to produce protected filler particles, which are then added in papermaking to improve the strength of the filled sheet. This patent also discloses that the calcium carbonate filler has a positive zeta-potential, produced by a pre-treatment of the filler with a small amount of a fully cooked cationic starch. The filler particles are made cationic by the addition of the starch with the objective to promote the adsorption of the anionic latex on the surfaces of filler particles. The latex-treated filler suspension, containing up to 20 parts of latex per 100 parts of chalk, is added before the headbox of the paper machine, for example, to the beater or pulper, and has a smaller negative effect on strength compared to untreated filler. Similarly, U.S. Pat. No. 4,445,970 discloses a method of manufacturing paper containing a mixture of clay and talc fillers and anionic latex to promote strength. The latex is preferably added to the machine chest, most preferably in amounts ranging between 3 and 7% based on the dry furnish.
At no point do any of the above patents disclose that the starch can be swollen in the presence of latex, either anionic or cationic, with or without the use of co-additives, for enhancing bridging between the swollen starch granules and the latex. Also, there are no references or claims related to the combination of swollen starch and latex in filler treatment for use in the manufacture of paper or as additives to the furnish used in papermaking.